InP-based high electron-mobility transistors (HEMT) and GaAs-based metamorphic electron-mobility transistors (MHEMT) with indium-rich channel designs are well known for their outstanding low noise as is disclosed in M. Schlectweg et al., 11th GaAs Symp., Munich, Germany, 2003 and high gain performance as is disclosed in D. Xu et al., IEEE Electron Device Lett., vol. 26, pp. 4-9, 2008, as a result of the superior transport properties associated with the InAlAs/InGaAs heterostructures. However, this materials system also limits the power performance of the HEMT because of its low breakdown and enhanced impact ionization. This problem becomes increasingly critical as the gate length is reduced to below 0.1 μm while the channel indium content is increased to boost gain for ultra-high-frequency operation. A need, therefore, exists for a way to address the low breakdown issue of ultra-short-gate high-gain HEMTs with indium-rich channel materials.
More particularly, it is desirable to improve the high frequency performance of InP-based HEMTs. For high frequency applications in excess of 300 GHz there are emerging sub-millimeter applications, for instance for transmitters operating at 340 GHz to enable imaging such as for instance for detection of explosives.
The conventional approach to enhancing high frequency performance including the incorporation of increased indium content in InGaAs channel layers and the reduction of gate length normally results in a reduction in breakdown voltages which limits the high power applications for such transistors. The use of wide band gap channel materials such as InP or InAsP can increase the breakdown voltage but only at the expense of the degradation of other important characteristics such as drain current and transconductance, making it difficult to generate sufficient gain at these ultra-high frequencies.